Description
Onset contains Ondansetron, a medicine widely used to prevent nausea and vomiting. It belongs to the class of drugs called antiemetics (5-HT3 receptor blockers). It works by blocking serotonin, a natural chemical that can trigger vomiting, especially during chemotherapy, radiotherapy, or after surgery. Doctors in Pakistan often prescribe Onset to patients undergoing cancer treatment, surgical recovery, or in cases of severe vomiting that cannot be managed with routine medicines.
Formula / Composition
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Active Ingredient: Ondansetron 8mg
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Drug Class: Antiemetic (5-HT3 receptor antagonist)
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Dosage Form: Tablet
Uses
Onset is commonly prescribed for:
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Nausea and vomiting caused by chemotherapy or radiotherapy
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Vomiting after surgery (post-operative nausea)
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Severe vomiting linked to infections (if recommended by doctor)
Precautions
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Do not use if you are allergic to Ondansetron or similar medicines.
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Inform your doctor if you have liver disease or heart rhythm problems.
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Use carefully in patients with electrolyte imbalance (low potassium or magnesium).
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Safe use during pregnancy only on doctor’s advice.
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Not recommended for children unless prescribed.
Warnings
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Ondansetron may prolong the QT interval (a heart rhythm change). Patients with congenital long QT syndrome should avoid it.
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Use with caution after abdominal surgery or chemotherapy, as it may mask signs of bowel blockage.
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It is not a substitute for gastric suction in cases of severe stomach obstruction.
Possible Side Effects
Most patients tolerate Onset well, but some may experience:
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Headache
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Constipation or diarrhea
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Drowsiness or dizziness
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Feeling of warmth or flushing
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Fever
Rare but serious effects:
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Chest pain, irregular heartbeat, or very low blood pressure
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Rash, itching, or severe allergic reaction (including anaphylaxis)
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Eye problems such as temporary blurred vision
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Muscle stiffness or unusual movements (rare extrapyramidal reactions)
Seek medical help if you notice chest discomfort, severe rash, difficulty breathing, or sudden dizziness.
Drug Interactions
Onset may interact with:
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Apomorphine (dangerous drop in blood pressure if taken together)
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Medicines for epilepsy (Phenytoin, Carbamazepine)
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Rifampin (reduces effectiveness of Onset)
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Tramadol (may reduce pain-relief effect)
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Other drugs that prolong the QT interval (some antibiotics, antidepressants, or antiarrhythmics)
Always inform your doctor about any other medicines you are taking.
When Not to Use
Do not use Onset if:
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You are allergic to Ondansetron or any of its components
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You are taking Apomorphine
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You have severe congenital heart rhythm problems (long QT syndrome)
How to Use
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Swallow the tablet whole with a glass of water.
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Can be taken with or without food.
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Always follow your doctor’s instructions on dose and timing.
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Do not take more than prescribed.
Storage & Disposal
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Store at room temperature, away from heat and sunlight.
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Keep in original packaging.
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Do not use expired tablets.
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Keep out of children’s reach.
Dr. Ahsan Malik, Oncologist Onset 8mg tablets for managing moderate to severe nausea, particularly in patients undergoing chemotherapy or recovering from surgery. The medication is effective in reducing nausea and enhancing comfort during treatment. While side effects like mild headaches or constipation may occur, they are generally well-tolerated. For patients with heart conditions, ECG monitoring may be advised. Overall, Onset 8mg is a safe and reliable option for alleviating nausea.
Medical Disclaimer: This information is for general knowledge only. Always consult a healthcare professional for guidance on using this medication.

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